This invention relates to a pollution abatement process and pollution abatement device which makes use of misting technology, together with cooling and condensation coils to effect targeted pollutants"" removal. It accomplishes these objectives with little wastewater generation and the use of reactants for specific pollutants. The invention eliminates the need for an exhaust stack, creating a huge savings for any facility from reduced stack maintenance costs, elimination of stack replacement costs and the elimination of boiler operations for stack warming.
General
According to the Principle of Environmental Control (cited by Marks"" Standard Handbook of Mechanical Engineers, Ninth edition), nature has provided two almost inexhaustible sumps for maintaining a steady-state environment on earth. The first of these is the 3 K background temperature of absolute space, which nature uses for heat rejection to close its heat balances. The second is the oceans, which serve to close the material balances of its cyclic processes by accepting the combined runoff of the continents. The greatest engineering progress comes when people control their environmental activities so as to take maximum advantage at minimum cost of these sumps and of nature""s cyclic process. This is the basic principle upon which the science of environmental control is founded.
Environmental control seeks to subdue and to utilize nature""s ecological cycles in order to serve people""s needs, thereby conserving natural energy and mineral resources, and to replenish desirable local flora and fauna populations by agriculture and cultivation to provide adequate food, clothing and shelter. Environmental control seeks to extend depletable fuel supplies with clean, abundant forms of reusable energy. Replenishable substitutes for other depletable resources are sought, as well as recycling means for scarce and irretrievable substances. Environmental control seeks to conserve land, air and water quality by diversion into adequately controlled air dispersion and drainage canals of concentrated runoffs. The public standard of living is highest when all these things are done voluntarily by a responsible, cost-conscious citizenry.
Products of Combustion
The combustion byproducts of hydrocarbon fuels primarily consist of nitrogen (N2), carbon dioxide (CO2), water (H2O), carbon monoxide (CO), unburned hydrocarbons (UHC), oxides of sulfur (SOx), particulate (soot) and oxides of nitrogen (NOx).
The last five items, CO, UHC, SOx, particulate and NOx are unwanted and undesirable. These pollutants are referred to as criteria pollutants and many regulatory agencies around the world have established guidelines for their control.
CO is a gas that is an intermediate product of combustion of hydrocarbon fuels.
UHC often results from poor fuel atomization or quenching of the combustion process by the combustion cooling air film or by high levels of water or steam injection.
Oxides of sulfur are formed when sulfur is present in the fuel during combustion. SOx forms over a wide range of combustion temperatures and cannot be controlled in the combustion process itself. Hence, SOx formation either must be prevented from occurring by limiting sulfur in the fuel or else the SOx that is created must be removed from the exhaust stream by wet scrubbing or sorbent injection.
Particulate matter (i.e. soot or smoke) results from the combustion of liquid fuels/air ratios in the combustion primary zone.
NOx, by virtually unanimous agreement, is considered a primary contributor to visible pollution and deteriorating air quality. The reduction of NOx has become the major focus of air quality regulations throughout the world in efforts to improve air quality around the world.
The present invention was created based on well known and proven technologies as described below.
Economizers
Economizers remove heat from the moderately low-temperature combustion gases after the gases leave the steam generating and superheating/reheating sections of the boiler unit. Economizers are, in effect, feed water heaters which receive water from the boiler feed pumps and deliver it at higher temperature to the steam generator. Economizers are used instead of additional steam-generating surface, since the feed-water and, consequently, the heat-receiving surface, are at temperatures below the saturated-steam temperatures. Thus, the gases can be cooled to lower temperature levels for greater heat recovery and economy.
Economizers are forced-flow, once through conversion heat transfer devices, usually consisting of steel tubes, to which feed-water is supplied at a pressure above that in the steam generating section and at a rate corresponding to the steam output of the boiler unit, they are classed as horizontal or vertical-tube type, according to geometrical arrangement; as longitudinal or cross flow, depending upon the direction of gas-flow with respect to the tubes;- as parallel or counter flow, with respect to the relative direction of gas and water flow; as steaming or non-steaming, depending on the thermal performance; as return-bend or continuous-tube, depending upon the details of design; and as base-tube or extended-surface, according to the type of heat- absorbing surface. Staggered or in-line tube arrangements may be used. The arrangement of tubes affects the gas flow through the tube bank, the draft loss, the heat transfer characteristics, and the ease of cleaning.
The size of an economizer is governed by economic considerations involving the cost of fuel, the comparative cost and thermal performance of alternate steam-generating or air heater surface, the feed-water temperature, and the desired exit gas temperature. In many cases, it is more economical to use both an economizer and an air heater.
Present day practice in individual independent economizers is to install economizers having a surface from 50 to 70% of the boiler heating surface. The main factor affecting heat transfer rates is the gas velocity over the economizer. With the ordinary economizers with gas velocities corresponding to approximately 1500 lb. of gas per square foot (71.700 N/m2) of gas passage area per hour through the economizers a transfer rate from 2.5 to 3.0 B.T.U. (2.65 kJ to 3.18 kJ) per hour per square foot (0.09 m2) of surface per xc2x0 F. (0.56xc2x0 C.) difference in temperature may be expected. If this velocity is increased to 3000 lb. (143,400 N/m2) per hour the rate will be from 3.5 to 4.5 B.T.U. (3.71 kJ to 4.77 kJ). Gas velocities through steel economizers are not high and transfer rates are low. With the latest designs of steel tube economizers where the counter flow principle is used, gas velocities are higher and heat transfer rates with this type of economizer will vary from 4 B.T.U. to 8 B.T.U. (4.24 to 8.48 kJ) at 300% of rating.
With the amounts of economizer surface ordinarily installed, the increase in boiler efficiency may be taken as 1% for each 10% of the boiler surface in the economizer at different ratings, and 1.4% for each 10% of surface at 300% of boiler rating.
The installation of economizers necessitates the use of an induced draft fan to overcome the increased draft resistance and also because of the reduction in gas temperatures. The draft loss due to an economizer is usually from 40 to 60% of the loss through to boiler to which it is attached. There are two types of flue gas economizers on which we based our theories: wet scrubber technology and flue gas to hydronic technology.
(i) Wet Scrubber Technology
Wet scrubbers or what is called static spray scrubbers are usually of the tower type, the gas passing upward counter-currently to the descending liquid. Sets of sprays are placed in the top zone, with various materials used in layer to channel and mix the gas and water. Hurdles, cylindrical tiles, and random packed ceramic tiles or metal spirals are common packing materials. With a gas flow-rate of about 350 ft3/min per ft2 (9.9 m3/min per 0.1 m2) of cross-sectional area and a water rate of 25 gal per Mcf (95 L per 28.3 m3) of gas, a cleanliness of 0.1 to 0.3 gr/ft3 (3.5 to 10.6 g/m3) can be obtained with blast furnace, cook oven, and producer gas. The pressure drop through a tower scrubber is in the range of 4 to 10 in. W.G. (7.5 mm Hg to 18.7 mm Hg). The scrubber process is used as the primary cleaning and cooling stages before the cleaning of gases. After scrubbing the gases the water droplets, by gravitational force empty into a steel reservoir at the bottom of the tower. Because of the acids that have been formed in the water, a separate nonmetallic piping system is installed with pumps, chemical treatment system and exchangers to circulate a low grade heat (approx. 120xc2x0 F. (48.80xc2x0 C.) to be used on existing heating equipment.
(ii) Flue Gas to Hydronic Technology
Conventional-type super heaters are installed by the boiler exit where the gas temperatures are lower than those in the zones where radiant-type super heaters are used. The tubes are usually arranged in the form of parallel elements on close lateral spacing and in tube banks extending partially or completely across the width of the gas stream, with the gas flowing through the relatively narrow spaces between the tubes. High rates of gas flow and, thus, high conventional heat-transfer rates are obtained at the expense of gas-pressure drop through the tube bank.
Super heaters, shielded from the boiler combustion zone by arches of wide-spaced screens of tubes, which receive heat by radiation from the high-temperature gases in cavities or inter tube spaces and also by convection due to the relatively high rate of gas flow through the tube banks, have both radiant and convection characteristics. Super heaters may utilize tubes arranged in the form of hairy loops connected in parallel to inlet and outlet headers; or they may be of continuous-tube type, where each element consists of a number of tube loops in series between the inlet and outlet headers. The latter arrangement persists the use of large tube bank, thus increasing the amount of heat-absorbing surface that can be installed and providing economy of space and reduction cost. Either type may be designed for the drainage of the condensate which forms within the tubes during outages of the unit. Both types require that every tube have sufficient internal fluid flow to prevent overheating during operation. The heat transferred from high-temperature gases by radiation and convection is conducted through the metal tube wall and imparted by convection to the high-velocity liquid or vapour in the tubes. The removal of heat by the liquid or vapour is necessary to keep the tube metals within a safe temperature range consistent with the temperature limits of oxidation and the creep or rupture strength of the materials. Allowable design stresses for various steels and alloy are established by the A.S.M.E. (American Society of Mechanical Engineers) code. For economic reasons, it is customary to use low-carbon steel in the inlet sections of the super heater, and, progressively, more costly alloys as the metal temperatures increase.
The rate of liquid or vapour flow through the tubes must be sufficiently high to keep the metal temperature within a safe operating range and to ensure good distribution of flow through all the elements connected in parallel circuits. This can be accomplished by arrangements which provide for multiple passes of liquid or vapour flow through the tube banks. Excessive liquid or vapour flow rates, while providing lower tube-metal temperatures, should be avoided, since they result in high pressure drop with consequent loss of thermodynamic efficiency. The spacing of the tubular elements in the tube bank and, consequently, the rate of gas flow and convection heat transfer are governed primarily by the types of fuel fired, draft loss considerations, and the fouling and erosive characteristics of fuel carried in the gas stream.
Fundamental Principles of Combustion
Combustion may be defined as that chemical process in which an oxidant is reacted rapidly with a fuel to liberate stored energy as thermal energy, generally in the form of high temperature gases. Small amounts of electromagnetic energy (light), electric energy (free ions and electrons), and mechanical energy (noise) are also released during the combustion process. Except for special applications, the oxidant for combustion is oxygen in the air.
Conventional hydrocarbon fuels contain primarily hydrogen and carbon, either in the elemental forms or in various compounds. Complete combustion of these fuel elements produces primarily carbon dioxide and water. However, small quantities of carbon monoxide and partially reacted flue constituents in the form of gases and liquid or solid aerosols may also form. Most conventional fuels also contain small quantities of sulfur, which is oxidized to SO2 or SO3 during the formation of mineral matter (ash), water, and inert gases which are released during the combustion process.
The rate at which a fuel is combusted is dependent on:
(1) the reaction rate of the combustible fuel constituents with oxygen;
(2) the rate at which oxygen is supplied to the fuel (mixing of air and fuel); and
(3) the temperature in the combustion region.
The reaction rate is fixed by the selection of the fuel. Increasing either the mixing rate or the temperature will increase the rate of combustion.
Complete combustion of hydrocarbon fuels is obtained when all of the hydrogen and carbon in the fuel is oxidized to water and carbon dioxide. Generally, to obtain complete combustion it is necessary to supply excess oxygen, or excess air, beyond that theoretically required to oxidize the fuel. Excess oxygen or excess air is usually expressed as a percentage of the air theoretically required to completely oxidize the fuel.
Stoichiometric combustion of a hydrocarbon fuel occurs when fuel is reacted with the exact amount of oxygen required to oxidize al of the carbon, hydrogen, and sulfur in the fuel to carbon dioxide, water and sulfur dioxide. Hence, the exhaust gas from stoichiometric combustion would contain no incompletely oxidized fuel constituents or oxygen. The percentage of carbon dioxide contained in the products of stoichiometric combustion is the maximum attainable and is referred to as the stoichiometric CO2, ultimate CO2, or maximum theoretical percentage of carbon dioxide.
Stoichiometric combustion, that is, combustion at zero excess oxygen without the formation of incompletely combusted fuel products, is seldom realized in practice. The need for economy and safety dictates that most types of combustion equipment must operate with some excess air. This assures that fuel is not wasted and that the combustion equipment will be sufficiently flexible in performance to provide complete combustion despite variations in fuel properties and in the rate in which fuel and air are supplied.
Combustion Reactions
Combustion reactions of oxygen with combustible elements and compounds in fuels occur in accordance with fixed chemical principles.
Oxygen required for combustion is normally obtained from air, which is a mechanical mixture of nitrogen and oxygen with small amounts of water vapour, carbon dioxide, and inert gases. For practical combustion calculations, it is considered that dry air consists of 20.95% oxygen and 79.05% inert gases (including nitrogen, argon etc.) by volume, or 23.15% oxygen and 76.85% inert gases by weight. For purpose of calculations, it is assumed that nitrogen passes through the combustion process unchanged, although it is known that small quantities of nitrogen oxides do form.
Combustion Calculations
Calculations of the quantity of air required for combustion and the quantity of flue gas products generated during combustion are frequently needed for sizing system components and as an input to efficiency calculations. Other calculations, such as values for excess air and theoretical CO2 are useful in estimating combustion system performance. Frequently, combustion calculations can be simplified by using molecular weight as the basis for the calculations. Molecular weights may be expressed in any units of weight. Thus, in English units, the pound molecular weight, or pound mole, is frequently used where the pound molecular weight of a compound is equal to the molecular weight of the compound expressed in pounds. A pound molecular weight of any substance contains the same number of molecules as a pound molecular weight of any other substance.
Corresponding to measurement standards common to the industries, calculations involving gaseous fuels are generally made on a volume basis, while calculations involving liquid and solid fuels are generally made on a weight basis.
Air Required for Combustion
Stoichiometric air or theoretical air is the exact quantity of air required to provide oxygen for complete combustion. The three most prevalent components in hydrocarbon fuels are completely combusted by the following reactions:
C+O2xe2x86x92CO2
H2+0.502xe2x86x92H2O
S+O2xe2x86x92SO2
C, H2, S and O in the above reactions can be taken to represent 1 lb. (1 kg) mole of carbon, hydrogen, sulfur and oxygen, respectively. Using approximate atomic weights (C=12, H=1, S=32 and 0=16), it is seen that 12 lb. (12 kg) of carbon are oxidized by 32 lb. (32 kg) of oxygen to form 44 lb. (44 kg) of CO2; 2 lb. of hydrogen are oxidized by 16 lb.(16 kg) of oxygen to form 18 lb. (18 kg) of water; and 32 lb. (32 kg) of sulfur are oxidized by 32 lb. (32 k) of oxygen to form 64 lb. (64 kg) of sulfur dioxides. These relationships can be extended to include other hydrocarbon compounds. The weight of dry air required to supply a given quantity of oxygen is 4.32 times the weight of the oxygen. The weight of oxygen or of air required to oxidize the fuel constituents were calculated based on standard tables (combustion reactions of common fuel constituents). Oxygen contained in the fuel, except that contained in ash, should be deducted from the quantity of oxygen required as this oxygen is already combined with fuel components. However, water vapour is always present in atmospheric air, and when the weight of air to be supplied for combustion is calculated, allowance should be made for water vapour.
As stated previously, combustion calculations are sometimes made on volumetric basis. Based on Avogdro""s Law, it can be shown that, for any gas, 1 lb. (1 kg) mole occupies the same volume, at a given temperature and pressure. Therefore, in reactions involving gaseous compounds, the gases react in volume ratios identical to the pound mole ratios. That is, for the oxidation of hydrogen in the above reaction, one volume (or 1 lb. (1 kg) mole) of hydrogen reacts with {fraction (1/2)} volume (or xc2xd lb. (1/2 kg) (mole) of oxygen to form one volume (or 1 lb. (1 kg) mole) of water vapour. The volume of air required to supply a given volume of oxygen is 4.79 times the volume of oxygen. The volumes of oxygen or of dry air required to oxidize the fuel constituents were calculated based on standard tables. Volume ratios were not given for fuels that do not exist in the vapour form at reasonable temperatures or pressures. Oxygen contained in the fuel should be deducted from the quantity of oxygen required, as this oxygen is already combined with fuel components. Allowance should be made for the water vapour which increases the volume of dry air by 1 to 3%. From the above relationships, the weight of dry air required for stoichiometric combustion of any hydrocarbon fuel may be obtained by the equation:
Pounds (kilograms) of dry air per pound (per kg) of Fuel=0.0144(8C+24H+3Sxe2x88x923O)
where C, H, S and O are the weight percentages of carbon, hydrogen, sulfur and oxygen in the fuel respectively. Analyses of gaseous fuels are generally reported on the basis of hydrocarbon components rather than on elemental content.
Quantity of Flue Gas Produced
The weight of dry flue gas produced per pound (kg) of fuel burned is required in heat loss and efficiency calculations. The weight of flue gas per pound (kg) of fuel is equal to the sum of the weight of fuel (minus ash retained in furnace), the weight of air theoretically required for combustion and the weight for excess air. For solid fuels the weight of flue gas produced may be determined from the flue gas analysis by:
Pounds (kg) of dry flue gas, per pound (kg) of fuel             11      ⁢              xe2x80x83            ⁢              CO        2              +          8      ⁢              xe2x80x83            ⁢              O        2              +          7      ⁢              (                  CO          +                      N            2                          )            xc3x97      C            3    ⁢          xe2x80x83        ⁢          (                        CO          2                +        CO            )      
Values for CO2, O2, CO, and N2 are percentages by volume from the flue gas analysis, and C is the weight of carbon burned per pound of fuel. Total dry gas volume of flue gases resulting from the combustion of one cubic foot (1 m3) of gaseous fuels for various percentages of CO2 may be determined by:       Dry    ⁢          xe2x80x83        ⁢    flue    ⁢          xe2x80x83        ⁢    gas    ,            cubic      ⁢              xe2x80x83            ⁢      feet      ⁢              xe2x80x83            ⁢      per      ⁢              xe2x80x83            ⁢      cubic      ⁢              xe2x80x83            ⁢      foot      ⁢              xe2x80x83            ⁢              (                              m            3                    ⁢                      xe2x80x83                    ⁢          per          ⁢                      xe2x80x83                    ⁢                      m            3                          )            ⁢              xe2x80x83            ⁢      of      ⁢                        xe2x80x83                ⁢                  xe2x80x83                    ⁢      fuel      ⁢              xe2x80x83            ⁢      gas        =                                                      Cubic              ⁢                              xe2x80x83                            ⁢              feet              ⁢                              xe2x80x83                            ⁢                              (                                  m                  3                                )                            ⁢                              xe2x80x83                            ⁢                              CO                2                            ⁢                              xe2x80x83                            ⁢              produced              ⁢                              xe2x80x83                            ⁢              per              ⁢                              xe2x80x83                            ⁢              cubic              ⁢                              xe2x80x83                            ⁢              foot                                                                                          (                                  m                  3                                )                            ⁢                              xe2x80x83                            ⁢              of              ⁢                              xe2x80x83                            ⁢              gas              ⁢                              xe2x80x83                            ⁢              burned              xc3x97              100                                                  Percent        ⁢                  xe2x80x83                ⁢                  CO          2                ⁢                  xe2x80x83                ⁢        by        ⁢                  xe2x80x83                ⁢        analysis            
The excess air quantity may then be determined by subtracting the quantity of dry flue gases which would result from stoichiometric combustion from the total volume of flue gas.
Sample Combustion Calculation